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Urinary System
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The
major function of this system is the elimination of excess water,
salts, and waste products.
Although
renal function declines substantially with age, it usually remains
sufficient for removing bodily wastes. Nevertheless, reduced renal
function decreases the elderly person's ability to respond to various
physiological and pathological stresses. In general, aging is
associated with an increased incidence of kidney problems. There is a
decline in the number of nephrons by about 30-40% between ages 25 and
85. Nephrons are the basic functional units of the kidneys and are made
up of a renal corpuscle and renal tubules. There is also a reduction in
filtration rate due to cumulative damage to the system and to reduction
in blood flow to the kidneys.
Urine is
formed by the kidneys through three processes: filtration, reabsorption
and secretion. The ultimate source of urine is blood, and a large
amount of blood is transported to the kidneys for filtration by way of
the renal arteries. Renal blood flow progressively decreases from 1200
mL/minute at age 30 to 40 years to 600 mL/minute at age 80. A decrease
in the glomerular filtration rate, or the rate at which blood is forced
through and filtered by the renal corpuscle in the kidneys, is the most
important functional defect caused by aging.
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Fun
fact:
A
typical adult kidney measures approximately 4 inches in length and 1.2
inches in thickness. This is about the size of a large bar of bath soap.
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Not
so fun fact:
Elderly
people on salt-restricted diets have a decreased ability to conserve
sodium. This can contribute to the increased tendency of elderly
persons to develop dehydration. Still, the most important cause of
dehydration is the loss of the sensation of thirst, a characteristic
common in the elderly. Loss of thirst is especially important when
elderly persons are confronted with an illness or an infection that
increases demands for or limits the intake of salt and water.
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Since the
kidneys receive up to 25% of the resting cardiac output through the
renal arteries, a large portion of total blood volume is filtered
through the renal corpuscle each day. At this rate, it
wouldn’t take long to totally deplete the body’s
entire blood volume. Since this is obviously not a common problem, the
body must have some way to recover a majority of that filtered blood.
The process that returns necessary items from the filtrate back into
the blood is called reabsorption.
Reabsorption
is responsible for returning 99% of filtrate back to the circulatory
system. Reduced nephron sensitivity which occurs with age, however,
results in less reabsorption of water, so urination becomes more
frequent while daily fluid requirements increase. At the same time,
there is usually a loss of thirst which compounds the problem. The
muscles that help regulate the release of urine become weaker which
leads to problems with incontinence and often involves slow leakage of
urine. This is usually a more common problem in women. In males,
urinary retention might develop due to chronic inflammation of the
prostate gland. Swelling of the gland prevents the flow of urine as it
puts pressure on the urethra.
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